St Asaph on the A55 is a pain in the butt and has been for a few years and when I went past last week it seemed as bad as ever.
You can be lucky however, but it depends on what time of the day your journey is.
I'm sorry Medz but there is no easy way round that problem as St Asaph is probably the best bet for your son.
With regard to the journey to Heathrow. I did that journey twice last year and the Sat Nav picked the quickest route as M56,M6,M1 and then the M25